This invention relates to a bipolar transistor and more particularly to such bipolar transistor having a so-called graft base region for base contact.
As a bipolar transistor with high operating speed and low power consumption, there in known one such as shown for example in FIG. 5. In this figure, an n-type semiconductor layer is formed on, for example, a p-type semiconductor substrate 1 by, for example, epitaxial growth, and isolated by isolation regions 2, such as silicon oxide, into several island-shaped regions, such as region 3. A p-type base region 4 is formed to confront to the surface of the n-type semiconductor layer of this island-shaped region 3 and an n.sup.+ type emitter region 5 is formed within this base region 4. This base region 4 is formed by a base active region 4i at substantially the central portion for achieving the function of the base per se and a so-called graft base region 4g of higher concentration for the base electrode contact. The graft base region 4g is formed by diffusion of the p-type impurities from a base contact electrode 6 of the p.sup.+ type polycrystal silicon layer, while the base active region 4i is formed by ion implantation of p-type impurities using the base contact electrode 6 as a mask. The emitter region 5 is formed by introduction of n-type impurities from the n.sup.+ type polycrystal silicon layer 7 which serves as an emitter contact. The n.sup.+ type polycrystal silicon layer 7 is formed on the base contact electrode and also covering a side wall portion of the layer 7. The inner edge of the insulating layer 8 is used for the emitter diffusion mask.
A base electrode 9B formed of such as A1 is connected to the base contact electrode 6, while an emitter electrode 9E is connected to the n.sup.+ type polycrystal silicon layer 7. It is noted that, in this example of FIG. 5, a collector electrode 9c is electrically connected to the island-shaped region 3 of the n-type epitaxial layer used as the collector region through the n.sup.+ type buried layer 11 and the n.sup.+ type electrode contact area 12.
With the above described so-called graft base type bipolar transistor, it is possible to simplify the process through emitter-base self-align and reduce parasitic capacity, while simultaneously minutizing the emitter width or achieving so-called shallow junction.
FIG. 6 shows to an enlarged scale the vicinity of the so-called graft base region 4g of the transistor. It may be seen from this figure that there exist a large quantity of the parasitic capacity C.sub.o through a depletion region D.sub.p in the vicinity of the bottom surface of the junction impeding high-speed operation. Also, when forming the isolation region 2 by, for example, selective oxidation of the silicon surface, fluctuations are caused in the element size due to so-called bird's beak generated in the vicinity of the semiconductor surface on the boundary of the area 2 and, above all, such fluctuations in the size are relatively increased when the area occupied by the element is to be decreased to achieve a higher degree of integration, thus impeding the higher degree of integration.
With this in view, a bipolar transistor having the structure shown in FIG. 7 is proposed. Thus a thin insulating layer 14 is formed on the semiconductor surface, and windows are formed in this insulating layer 14 at a prescribed distance from the boundary of the isolation region 2, after which the base contact electrode 6 formed of the aforementioned P.sup.+ type polycrystal silicon is deposited. This predetermined distance is so set as to be able to avoid the effect of the so-called bird's beak on the boundary of the aforementioned isolation region 2. Since the structure is otherwise the same as that in the above described example of FIG. 5 so that the corresponding parts are indicated by the same reference numerals and the corresponding description is omitted.
It is possible with the bipolar transistor of FIG. 7 to reduce the base area with higher precision and to reduce the parasitic capacity.
However, a depletion region D.sub.p exists which encircles the graft base region 4g, as shown in FIG. 8. Therefore, when considering the collector-base capacity there are produced a capacity C.sub.o ' on the side of the junction bottom surface, a capacity C.sub.s on the side of the junction lateral surface and a capacity C.sub.ox formed by the base contact electrode 6 and the collector region (n-type epitaxial layer) 3 through an insulating layer 14, so that the parasitic capacity may become larger than that of the structure of FIG. 5 depending on the size.